A method to allow the prediction of the date of maize (Zea mays L.) maturity early in the development of the plant was developed in order to evaluate the feasibility of planting a second crop in a sequential cropping system. Maize (Dekalb XL71) was planted on 15 planting dates in 1981 and 1982. Three maize phenological events, emergence, tasseling and harvest maturity, were recorded in terms of 'days to event'. When days to event was regressed on planting date (days Julian), the slopes were significantly different (P>0.0001) from one another; orthogonal comparisons showed that the slopes for tasseling and harvest were not significantly different (P=0.2739), but both differed significantly (P>0.0001) from that of emergence. This means that XL71 maize harvest maturity could be predicted and used in a decision-making process for the sequential cropping of soybean and sorghum by the addition of 58 days to time of 50% tasseling in northern Florida. A modified growing degree unit base has been established for Dekalb XL71 in the SE United States. When modified growing degree units (MGDU's) to event (emergence, tasseling or maturity) were regressed on planting date, the three slopes were not statistically different. Thus MGDU's from planting date, emergence and 50% tasseling can be used to predict maize harvest maturity.